The aim of this study was to determine if severe malnutrition affects the proportion of proliferating bone marrow cells in malnourished rats during lactation. Sister chromatid staining of metaphases was used as a parameter, as well as the morphology, size and color of bromodeoxyuridine labeled interphase nuclei. The BrdU proliferation labeling index was statistically lower in malnourished rats (20.4%), than in well-nourished controls (35.1%). A difference was also found between the two groups in the proportion of metaphases in first, second and third or successive cell cycle. The average generation time was longer in the malnourished group: 15.3 h, against 11.8 h for the controls. These results indicate that severe malnutrition affects both the proportion of bone marrow proliferating cells and their cell kinetics.